1. Field of the Invention
The present invention relates to a fuel cell including a membrane electrode assembly and a pair of metal separators sandwiching the metal separators. The membrane electrode assembly includes a solid polymer electrolyte membrane, a first electrode provided on one surface of the solid polymer electrolyte membrane, and a second electrode provided on the other surface of the solid polymer electrolyte membrane. The first electrode has a first electrode catalyst layer and a first gas diffusion layer, and the second electrode has a second electrode catalyst layer and a second gas diffusion layer. The surface size of the first electrode catalyst layer is smaller than the surface size of the second electrode catalyst layer.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a solid polymer electrolyte membrane. The solid polymer electrolyte membrane is a polymer ion exchange membrane, and interposed between an anode and a cathode to form a membrane electrode assembly (MEA). The membrane electrode assembly and a pair of separators sandwiching the membrane electrode assembly make up a power generation cell for generating electricity. In use, typically, a plurality of the power generation cells are stacked together to form a fuel cell stack, and the fuel cell stack is, e.g., mounted in a vehicle for use as a fuel cell system.
In some cases, the membrane electrode assembly adopts structure where the surface size of one of the electrodes is smaller than the surface size of the solid polymer electrolyte membrane, and the surface size of the other of the electrodes is the same as, or smaller than the surface size of the solid polymer electrolyte membrane, and larger than the surface size of the one of the electrodes (stepped-type MEA).
For example, in a fuel cell disclosed in Japanese Patent No. 3242737, as shown in FIG. 31, an electrode 2 is provided on one surface of an electrolyte membrane 1, and an electrode 3 is provided on the other surface of the electrolyte membrane 1. The surface size of the electrode 2 is smaller than the surface size of the electrode 3. At the outer end of the electrolyte membrane 1, a spacer 4 is provided around the electrode 2, and a spacer 5 is provided around the electrode 3.
The electrodes 2, 3 have different surface areas. Further, ends of the electrodes 2, 3 are not positioned symmetrically with respect to the electrolyte membrane 1. Therefore, according to the disclosure, in the peripheral areas of the electrodes 2 and 3, any of the electrodes 2 and 3 supports the electrolyte membrane 1, and improvement in the mechanical strength in the peripheral areas is achieved.